MXPA01011447A - Methods for thermal mass transfer printing. - Google Patents

Abstract

A method of thermal mass transfer printing a colorant including a binder media from a ribbon onto a first surface of a web having a non-homogeneous thermal conductivity, a non-planar printing surface, a non-homogeneous structure or chemical incompatibility. The first surface of the web is preheated prior to thermal mass transfer printing. The surface of the ribbon containing the colorant is positioned opposite the first surface of the heated web at an inner face. A thermal print head is positioned at the interface on the side of the ribbon opposite the colorant. The web is moved relative to the thermal print head. Printing is completed by selectively applying localized heat to the ribbon from the thermal print head and pressure at the interface to cause the transfer of colorant from the ribbon to the heated web.

Description

METHOD AND SYSTEM FOR PRINTING THERMAL MASS TRANSFER FIELD OF THE INVENTION The present invention is concerned with an improved process for thermal mass transfer printing on substrates and in particular with the preheating of the substrate to compensate for differences in thermal conductivity, topography superficial and / or chemical incompatibility.

BACKGROUND OF THE INVENTION Thermal printing is a term widely used to describe several different families of technologies for making an image on a substrate. These technologies include hot stamping, direct thermal printing, dye diffusion printing and thermal mass transfer printing. Hot stamping is a mechanical printing system in which a pattern is stamped or embossed through a ribbon on a substrate, as disclosed in U.S. Patent No. 4,992,129 (Sasaki et al.). The pattern is printed on the substrate by applying heat and pressure to the pattern. A colored material on the Re .: 133956 Tape, such as a dye or ink, is transferred by this to the substrate where the pattern has been applied. The substrate can be preheated before printing the pattern on the substrate. Since the stamp pattern is soft, hot stamping can not easily be used to apply indicia or variable images on the substrate. Consequently, hot stamping is not normally used to print variable information, such as printing sheets used to make sheets of 10 enrollments. Direct thermal printing was commonly used in old facsimile machines. Those systems required a special substrate that includes a dye in such a way that localized heat can change 15 the color of the paper in the specific place. In service, the substrate is transported further than an array of small individual heating elements or pixels that heat (or do not burst) the substrate. Whenever the pixels heat the substrate, the substrate changes color. To the By coordinating the heating action of the pixels, images such as letters and numbers can be formed on the substrate. However, the substrate may change color unintentionally such as when exposed to light, heat or mechanical forces. 25 Thermal transfer by diffusion of dye involves the transport of dye through the physical diffusion process fa dye donor layer to a dye receptor substrate. Similar to direct thermal printing, the ribbon containing the dye and the substrate is transported further than an array of heating elements (pixels) that selectively heat the ribbon. Whenever the pixels heat the ribbon, the solid dye is liquefied and transferred to the substrate via diffusion. Some known dyes interact chemically with the substrate after being 10 transferred by diffusion of the dye. The formation of color on the substrate may depend on a chemical reaction. Consequently, the color density may not fully develop if the thermal energy (the temperature reached or the elapsed time) is too low. So, the development 15 or color development using dye diffusion is frequently enhanced by a post-printing step such as thermal fusion. Alternatively, U.S. Patent No. 5,553,951 (Simpson et al) describes one or more upstream temperature control rolls or 20 downstream to provide greater temperature control of the substrate during the printing process. Thermal mass transfer printing, also known as thermal transfer printing, non-impact printing, thermal graphic printing and 25 thermography, it has become popular and commercially successful ffJJA by to form characters on a substrate. Like hot stamping, heat and pressure are used to transfer an image from a ribbon onto a substrate. As direct thermal printing and dye diffusion printing, pixel heaters selectively heat the ribbon to transfer the dye to the substrate. However, the dye on the ribbon used for thermal mass transfer printing includes a polyimic binder, usually composed of wax and / or resin. So, when the element of When the pixel heating heats the tape, the resin wax mass is transferred from the tape to the substrate. A problem with thermal mass transfer printing is to produce high print quality on non-compatible surfaces, such as non-flat surfaces or 15 rough, surfaces with non-uniform thermal conductivity and when the composition of the substrate is not chemically compatible with the binders in the colorant. Figure 1 illustrates an example of a substrate 20 having both a rough or non-smooth printing surface 20 22 and an inhomogeneous thermal conductivity. The retroreflective laminate 20 includes a plurality of glass beads 24 attached to a support 26 by a resin / polymer matrix 28. In the illustrated embodiment, a retroreflective layer 29 is interposed between the support 26 and the support 26. . * jjft * tf **. Atheal i, if resin / polymer matrix 28. The glass beads 24 protrude from the resin / polymer matrix 28 normally by an amount from about 1 miera to about 5 micras, forming a rough or non-planar surface for printing by thermal mass transfer. Since the retroreflective laminate 20 is not constructed of a single homogeneous material, the thermal conductivity along the printing surface 22 may vary. For example, the thermal conductivity of the glass beads may be different from the thermal conductivity of the resin / polymer matrix 28. In addition, the thermal conductivity may be affected by varying the thickness of the support 26, gaps or voids in the support 26 or mounds or stacks of glass beads 24 on the retroreflective laminate 20. Consequently, the application of an image to the printing surface 22 using conventional thermal mass transfer printing techniques can result in a variable thickness in the thermal mass transfer 23 and / or variable adhesion of dye pixel dots, with a corresponding degradation in print quality. Figure 2 illustrates an alternative substrate having a printing surface 30 with variable thermal conductivity. Figure 2 illustrates a sealed or encapsulated retroreflective laminate 32. Microspheres or Glass beads 34 are glued to a gluing layer 36 with an optional reflective layer 38 interposed therebetween. A protective layer 40 is attached to the tie layer 36 by a plurality of raised supports 42. The protective layer 40 forms a space 44 above the microspheres 34. Consequently, the thermal conductivity of the printing surface 30 varies significantly between the regions on the spaces 44 and regions on the raised supports 42. Typically, the thickness and coverage percentage of a thermal mass transfer layer 46 varies between the regions on the spaces 44 and the regions on the raised supports 42. The figure 3 illustrates an example of a sealed or encapsulated retroreflective laminate in which the raised supports form a hexagonal pattern on the printing surface. Due to the variation in the thermal conductivity of the printing surface, the hexagonal pattern of the raised support is shown through the printed image on the retroreflective laminate of Figure 3. U.S. Patent Nos. 5,818,492 (Look) and 5,508,105 (Orensteen et al.) teach that thermal mass transfer printing can be performed on a retroreflective laminate in those instances where there is a polymer layer or layers disposed thereon. In so much that the addition of a polymeric layer has improved printing capacity on some retroreflective laminates, the process of adding the layer increases the cost of the final product and can degrade the retroreflective properties of the substrate. Even with the additional layer, the print quality is not appropriate for some graphic applications. The addition of a printable layer can alter other characteristics of the retroreflective laminate, such as frangibility. JP-A-05-270044 discloses a method of recording or recording by thermal transfer that transfers an image to the receiver by heating the thermal transfer recording medium with the heating means for image transfer, wherein the receiver is previously heated at the time of heating the recording medium by thermal transfer with the heating means for image transfer. JP-A-07-227977 discloses an image transfer method provided with an intermediate sheet having a layer of light transmitting dye on one side of the surface of a web, means for selectively recording or recording an image to the layer dyeing or dyeing means for allowing the surface of the dye layer of the intermediate sheet to come into contact with an image receiving element and means for transferring the dye layer onto the dye layer. image receiving element by means of heat and pressure. This method comprises a step of preheating the intermediate sheet before heating and pressurizing. In order to use thermal mass transfer printing on an unsupported surface, the most common methods to improve print quality is to increase the thermal energy of the print head and increase the pressure applied to the print head by the backup roller. However, increasing the thermal energy and pressure can lead to a decreased printer head life, wrinkling of the tape, lower print quality and mechanical stresses in the printing system. Accordingly, what is needed is a method and apparatus for thermal mass transfer printing on substrates having a roughened surface, inhomogeneous thermal conductivity and / or a surface composition that is not immediately compatible with the dye of the printing ribbon. by thermal mass transfer.

BRIEF DESCRIPTION OF THE INVENTION The present invention is concerned with a method and apparatus for preheating the substrate at a certain temperature, depending on the particular substrate and the colorant to be used, in order to increase the thermal energy of the substrate surface to improve the print quality to a thermal energy of the print head low and pressure in a thermal mass transfer printing system. The present method and apparatus extends the field of thermal mass transfer material / ribbon combinations that are useful for thermal mass transfer printing. The present method is suitable for tapes having a non-planar printing surface, such as an unsealed retroreflective laminate, inhomogeneous thermal conductivity, such as a sealed or unsealed retroreflective laminate or a surface that is chemically incompatible with the binder in the Colorant. In one embodiment, the apparatus includes a heater or heating element positioned within the printer chassis by thermal mass transfer near the print head in the direction of the fabric. As the fabric moves, the heater directs the radiant energy over the substrate, heating the surface and making it more receptive to the printed image. The apparatus preferably includes a uniform transverse fabric heating which is adjustable via a specialized external control or via an interface with the image generating computer. The output of the heater is normally adjusted to the minimum level necessary to obtain a Optimal print quality. In multi-head printers, a similar heating element can optionally be positioned upstream of each print head. The apparatus may optionally be equipped with a radiant heater and thermal shield shutter to allow an instantaneous on / off cycle. In one embodiment, the shutter is a Venetian blind structure that can be opened and closed to intermittently expose the fabric to the source of radiant heat. In one embodiment, the method for thermally transferring a dye including a binder means of a ribbon onto a first surface of a fabric having a nonhomogeneous thermal conductivity (thermal capacity) includes preheating the first surface 15 of the fabric before printing by thermal mass transfer. The surface of the tape containing the colorant is positioned opposite the first surface of the fabric heated at an interface. The thermal print head is positioned at the interface on the opposite side of the tape 20 to the colorant. The fabric is moved relative to the thermal print head. Printing is accomplished by selectively applying localized heat to the thermal printhead ribbon and pressure at the interface to cause transfer of dye from the ribbon to the heated fabric. In another embodiment, the present invention includes the positioning a plurality of thermal printheads in a plurality of respective interfaces opposed to the dye on the tapes. In one embodiment, the first surface of the fabric is preheated before coupling with each of these interfaces. In a modality with multiple print heads, tapes with different dyes can be used in each of the print heads.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS Figure 1 is a side sectional view of an image formed on a retroreflective laminate of pearls using conventional thermal mass transfer printing. Figure 2 is a side sectional view of an image formed on a sealed retroreflective laminate using conventional thermal mass transfer printing. Figure 3 is an image formed on a sealed retroreflective laminate using conventional thermal mass transfer printing. Figure 4 is a schematic illustration of a thermal mass transfer printer according to the present invention. Figure 5 is a side sectional view of a laminate of exposed beads having a thermal mass transfer image applied in accordance with the method of the present invention. Figure 6 is a side sectional view of a sealed retroreflective laminate having a thermal mass transfer image applied in accordance with the method of the present invention. Figure 7 is a side sectional view of an alternative sealed retroreflective laminate having a thermal mass transfer image applied in accordance with the method of the present invention. Figure 8 is an exemplary image formed on a sealed retroreflective laminate applied in accordance with the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Dye refers to a binder means of wax, resin or a combination thereof containing pigments and / or dyes that is capable of providing an image or indicia on the surface of the fabric. Thermal mass transfer printing refers to those processes that transfer dye from a ribbon to a substrate through the simultaneous application of localized heat and pressure. Tape refers to a carrier fabric that has a layer of dye on a surface. Incompatibility Chemistry generally refers to low adhesion of the colorant, lack of surface penetration between the colorant and the fabric, and wetting of the colorant during thermal mass transfer printing, thereby increasing the percentage of voids in the printed image. Figure 4 is a schematic illustration of a thermal mass transfer printer 50 according to the present invention. The print head 52a is positioned to engage a first side 68 of a moving web 54 as it passes through the printer by thermal mass transfer 50. A thermal mass transfer ribbon 56a is fed to an interface 58a between the print head 52a and the moving fabric 54. In the illustrated embodiments, the thermal mass transfer ribbon 56a is tensioned transversely to the print head 52a by a supply spool 60a and a winding spool 62a. A backing roller 64a is located along the opposite side of the fabric 54 to maintain pressure at the interface 58a. The fabric 54 is conveyed in the direction 66 by known mechanisms, such as a friction drive using a stepper motor. The print head 52a remains stationary and makes contact with the thermal mass transfer belt 56a and transfers the dye from the belt 56a to the first side 68 of the moving web 54. When the dye transfer is completed or not to be applied, the print head 52a and the thermal mass transfer belt 56a can optionally be retracted from the moving web 54 along an axis 70. A heater or heating element 72 is located upstream of the print head 52a. In the illustrated mode, the heater is a hot can roll 73. The amount of cloth 54 that is wrapped around the hot can roll 73 may vary depending on the application. For some applications, the hot can roller 73 is polished and / or includes a Teflon® plasma coating to prevent the fabric 54 from sticking at higher temperatures. The hot can roller 73 is heated by a conventional electric tube heater that is kept stationary while the hot can 73 rotates. The hot can roller 73 can be mounted by bearings in such a manner that it rolls freely with the moving web 54. In the illustrated embodiment, the heater is rated at 2400 Watts or approximately 79 Watt / cm (200 Watts per inch). Alternative heaters include convection heaters, UV heaters, microwave generators, RF generators, hot lamps and the like. The thermal mass transfer printer 50 of Figure 4 includes four print heads 52a, 52b, 52c, 52d and the associated structure. In an alternative embodiment, additional heaters 74b, 74c, 74d are located upstream (based on the travel directions of the fabric 66) of the thermal printheads 52b, 52c, 52d. In the illustrated embodiment, the additional heaters 74b, 74c, 74d are hot lamps. In the embodiment in figure 4, indicia or images of more than one color can be applied to the moving web 54. A color print or four-color process can be obtained by using thermal mass transfer tapes with black, magenta dye , cyan and yellow as the transparent color overlaps each of the print heads 52a, 52b, 52c and 52d. The thermal print head 52a, 52b, 52c and 52d is put into operation to transfer discrete dye areas to the first side 68 of the fabric 54. The size of the dye transfer area or dot can be determined by the area of each item Warmed discreetly over the print heads. Such points are in general about 0.006 square millimeters, which is the area of a single pixel. The resolution of the indicia printed by the print heads 52a, 52b, 52c and 52d is generally from about 75 to about 250 dots per linear centimeter.

The term "thermal print head" refers to the mechanism or mechanisms by which a localized heat for dye transfer is generated. The localized heat can be generated by resistance elements, elements that come into contact with the tape in a laser system, electronic elements, thermally activated valve elements, inductor elements, thermopile elements and the like. An example of a printhead that can be incorporated into the thermal mass transfer printer 50 of FIG. 4 is the printhead incorporated into an apparatus sold under the tradename Model Z170, manufactured by Zebra Technologies Corporation of Vernon Hills, Illinois. . The thermal mass transfer belt 56a, 56b, 56c and 56d may have a wax base, a resin base or a combination of wax-based binder and resin. Commercially available tapes suitable for use in the thermal mass transfer printer 50 of Figure 4 are available under the tradename Zebra by Zebra Technologies Corporation, model number 5030, 5099 and 5175. These thermal mass transfer tapes typically include a polyester support about 6 microns thick and a dye layer from about 0.5 microns to about 6.0 microns thick. Additional revelations concerning the techniques of conventional thermal mass transfer printing are summarized in U.S. Patent Nos. 5,818,492 (Look) and 4,847,237 (Vanderzanden). Figure 5 is an enlarged cross-sectional view of the retroreflective laminate 20 of Figure 1 having an image 100 formed on the non-planar printing surface 102 using the thermal mass transfer method and printer apparatus of the present invention. A non-planar printing surface refers to a roughness or surface roughness of at least 1 micron to about 5 microns. A sealed retroreflective laminate may have a surface roughness of about 10 microns to about 15 microns. The retroreflective laminate 20 also has an inhomogeneous structure as measured along a vertical axis and voids in the resin / polymer matrix 28 that bonds the beads to the support 26. As illustrated in FIG. 1, the Thermal mass transfer printing forming the image 100 has a generally uniform adhesion of the thermal mass to the retroreflective laminate 20. Figure 6 is a side sectional view of a sealed retroreflective laminate having a printing surface 110. The combination of the raised supports 112 and the spaces 114 result in a non-uniform thermal conductivity and non-uniform thermal capacity through and b of the printing surface 110, as measured along an axis normal to the printing surface 110. The present method and apparatus for thermal mass transfer printing results in a printed layer by thermal mass transfer substantially uniform 116 despite non-uniformity in thermal conductivity. Figure 7 is a side sectional view of a sealed retroreflective laminate 120 having a printing surface 122 that is non-planar and has a 10 non-uniform thermal conductivity and non-uniform thermal capacity. As discussed above, raised supports 124 and spaces 126 result in non-uniform thermal conductivity across the printing surface 122. The uneven surface created by the 15 cubic corner elements 125 also contributes to non-uniformity of thermal conductivity. Additionally, the application process of the sealing film 128 results in depressions or sealed lines 130 through the printing surface 122. Despite these two 20 disadvantages, the present method and apparatus provides a substantially uniform thermal mass transfer printing layer 132 through the printing surface 122. FIG. 8 illustrates a logo printed on a 25 sealed retroreflective laminate using the method and thermal mass transfer printing apparatus of the present invention. Contrary to the results shown in Figure 4, the present method and apparatus result in a substantially uniform image despite the hexagonal sealed lines 5 and corresponding non-uniformity of the thermal conductivity. The present method and apparatus for thermal mass transfer printing can be used to produce alphanumeric characters, graphic images, codes of 10 bars or the like. The fabric can be a sealed or unsealed retroreflective laminate, for example a cubic corner laminate disclosed in U.S. Patent Nos. 3,684,348, 4,801,193, 4,895,428 and 4,938,563; or a pearl lens laminate comprising a 15 exposed lens element, encapsulated lenses or buried lenses as disclosed in U.S. Patent Nos. 2,407,680, 3,190,178, 4,025,159, 4,896,943, 5,064,272 and 5,066,098.

EXAMPLES Example 1 A series of paired pairs of printed samples were prepared using a thermal mass transfer printer illustrated in general in Figure 4 with and without 25 preheating the fabric before printing. All Samples were printed by thermal mass transfer with a DC300 blue sapphire thermal mass transfer ribbon available from IIMAK Corp. of Amhurst, NY. Then the percent of holes in the solid image generated was evaluated. The fabrics moved through the printer at a line speed of approximately 7.62 cm / second (3 inches / second). The same image and thermal energy was applied to the fabrics during printing. For those samples that were preheated, the preheat temperature fluctuated from approximately 76.7 ° C to approximately 93.4 ° C (170 ° F to 200 ° F) as indicated in Table 1. Fabric samples A, B, I, J , O and P consisted of laminates of Scotchlite retroreflective sheet metal plates, Series 3750 of Minnesota Mining and Manufacturing Company of St. Paul, Minnesota, with a topcoat of polyvinyl chloride-vinyl acetate-plasticized vinyl alcohol terpolymer. Cloth samples C and D consisted of laminates of Scotchlite retro-reflective sheet metal plates, Series 4770A from Minnesota Mining and Manufacturing Company of St. Paul, Minnesota, with a cross-linked aliphatic urethane top coating. Samples of cloth E and F consisted of a retroreflective laminate of High Intensity Grade Scotchlite, Series 3870 of Minnesota Mining and Manufacturing Company, St.

Paul, Minnesota, with an acrylic top coating. Fabric samples G and H consisted of a Scotchlite Diamond Grade laminate, Series 3970 from the Minnesota Mining and Manufacturing Company of St. Paul, Minnesota, with an acrylic top coating. Samples of cloth K and L consisted of laminates of retroreflective Scotchlite Series 3750 plates, with an exposed surface of polyvinyl butyral and exposed glass beads. The fabric samples M and N consisted of a laminate of retroreflective Scotchlite license plates, Series 3750, with a cross-linked aliphatic urethane topcoat. The fabric samples Q and R consisted of laminates of Scotchlite retro-reflective plate plates, Series 3750 with a top coating of aliphatic polyester urethane. The fabric samples S and t consisted of a laminate of Scotchlite retro-reflective number plate plates, Series 4770A, with a top coating of ethylene-extruded acrylic acid copolymer.

Hundreds reduction in solid image from about 55% to 95.6%. The most spectacular visual improvement in image quality appears in samples E and F. Samples C and D are probably the most 15 difficult to print by transfer of thermal mass due to the chemical incompatibility of the fabric and the thermal mass on the belt. The preheating of the. fabric results in a 78.8% reduction in voids in the solid image. The laminate with exposed lens beads of the 20 samples K and L exhibit the highest surface roughness. The preheating results in a percentage reduction of gaps in the solid image of approximately 60%. As several modalities of the present 25 invention have now been described, it will be evident to those of ordinary skill in the art that various changes and modifications may be made without deviating from the concept of the invention summarized above. Thus, the scope of the present invention should not be limited to the structures described in this application, but only by the structures described by the language of the claims. It is noted that, with regard to this date, the best method known to the applicant to carry out the aforementioned invention is that which is clear from the present description of the invention.

Claims (9)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A method of printing by thermal mass transfer of a dye from a ribbon onto a first surface of a fabric, characterized in that it comprises the steps of : preheating the first surface of the fabric to form a heated fabric, the first surface comprising one or more of a non-planar surface, a surface with inhomogeneous thermal conductivity, and a surface chemically incompatible with the colorant; positioning a surface of the tape containing the dye opposite the first surface of the heated fabric at an interface; positioning a thermal print head at the interface on one side of the ribbon opposite the colorant; moving the fabric in relation to the thermal print head and selectively applying heat and pressure located to the thermal print head ribbon at the interface to cause transfer of the dye from the ribbon to the heated fabric. 2. The method of compliance with the claim 1, characterized in that the fabric comprises an unsealed retroreflective laminate. 3. The method according to claim 1, characterized in that it comprises the step of moving the fabric beyond a stationary thermal print head. 4. The method according to claim 1, characterized in that it comprises the step of positioning a plurality of thermal printheads in a plurality of respective interfaces. The method according to claim 1, characterized in that it comprises the steps of: positioning a plurality of thermal printing heads in a plurality of interfaces and heating the first surface of the fabric before moving the fabric to each of the plurality of interfaces. The method according to claim 1, characterized in that it comprises the steps of: advancing the fabric beyond a plurality of stationary thermal print heads and locating a heat source upstream of each thermal print head. 7. The method according to claim 1, characterized in that it comprises the step of positioning a surface of a plurality of tapes containing the dye opposite the first surface of the fabric < 3H heated in a plurality of respective interfaces formed with a plurality of thermal printheads 5 corresponding. The method according to claim 7, characterized in that two or more of the tapes contain dyes having different colors. The method according to claim 10 1, characterized in that the fabric comprises a sealed retroreflective laminate.